Geological Society, 151 



and, as a final mean of these three, 14* 1932 as the number represent- 

 iny the density of solid mercunj at its fiisiiig-point as referred toivater 

 at 4° C. taken as unity. I think this result (which, it will be seen, 

 differs considerably from the figures hitherto quoted) may be fairly 

 accepted with confidence. 



In these experiments most of the weighings were made by 

 Adjunct Professor Dunnington, and the freezing-mixtures were 

 managed, at no small cost of personal discomfort, by Messrs. Bryan 

 and Memminger, students in this Laboratory. To these gentlemen 

 my thanks are due. 



GEOLOGICAL SOCIETY. 



[Continued from p. 77.] 



June 20th, 1877.— Prof. P. Martin Duncan, M.B., F.E.S., President, 



in the Chair. 



The following communications were read : — 



1. " On a hitherto unnoticed circumstance affecting the piling- 

 up of Volcanic Cones." By R. Mallet, Esq., F.E.S., P.G.S. 



After some remarks upon the two forms of volcanic activity, the 

 earlier system of " fissure eruption," and the present one of " erup- 

 tion at explosive foci," which he did not think could be carried 

 back much beyond the Tertiary epoch, the author discussed the 

 ordinary method of formation of a volcanic cone, and pointed out 

 that the effect of the piling-up of material must produce a pressure 

 on the original surface commensurate with the amount of material 

 heaped up on it, and therefore increasing gradually from the cir- 

 cumference nearly to the centre of the cone, where the loftiest 

 column of material presses upon the unit of space. When the sup- 

 porting rock is unyielding, such as the granite which bears thePuys 

 of Auvergne, it will probably maintain its original position ; but 

 when it is of a more yielding nature, as in the case of the ordinary 

 stratified rocks, the pressure of the cone will produce a saucer- 

 shaped depression, deepest in the centre where the greatest pressure 

 occurs ; and this tendency to sink will be aided materially by the 

 honeycombing and evisceration of the subjacent rock-masses ex- 

 posed to the action of the volcano. The consequence of this de- 

 pression of the surface supporting the cone will be to diminish 

 the original slope of the successive superimposed deposits, and 

 even in some cases cause the lowest beds to slope from the circum- 

 ference towards the centre. If the strata upon which the volcano 

 stands be particularly plastic, its pressure may cause an uprise of 

 the strata into protuberances round the foot of the mountain. 

 Sioiilar phenomena may occur when the support of the cone is 

 formed by older volcanic deposits. 



2. " The Steppes of Southern Russia." By Thomas Belt, Esq., 

 F.C.S. 



The author describes sections of strata in the south of Russia, and 

 traces the following succession of events : — 



